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SOCKET(2)										SOCKET(2)

NAME
       socket - create an endpoint for communication

SYNOPSIS
       #include <sys/types.h>
       #include <sys/socket.h>

       s = socket(domain, type, protocol)
       int s, domain, type, protocol;

DESCRIPTION
       Socket creates an endpoint for communication and returns a descriptor.

       The  domain  parameter  specifies  a communications domain within which communication will
       take place; this selects the protocol family which should be used.   The  protocol  family
       generally is the same as the address family for the addresses supplied in later operations
       on the socket.  These families are defined in the include file <sys/socket.h>.	The  cur-
       rently understood formats are

	      PF_UNIX	  (UNIX internal protocols),
	      PF_INET	  (ARPA Internet protocols),
	      PF_NS	  (Xerox Network Systems protocols), and
	      PF_IMPLINK  (IMP "host at IMP" link layer).

       The  socket  has the indicated type, which specifies the semantics of communication.  Cur-
       rently defined types are:

	      SOCK_STREAM
	      SOCK_DGRAM
	      SOCK_RAW
	      SOCK_SEQPACKET
	      SOCK_RDM

       A SOCK_STREAM type provides sequenced, reliable, two-way connection  based  byte  streams.
       An out-of-band data transmission mechanism may be supported.  A SOCK_DGRAM socket supports
       datagrams (connectionless, unreliable  messages	of  a  fixed  (typically  small)  maximum
       length).   A  SOCK_SEQPACKET socket may provide a sequenced, reliable, two-way connection-
       based data transmission path for datagrams of fixed maximum  length;  a	consumer  may  be
       required  to  read an entire packet with each read system call.	This facility is protocol
       specific, and presently implemented only for PF_NS.  SOCK_RAW sockets  provide  access  to
       internal network protocols and interfaces.  The types SOCK_RAW, which is available only to
       the super-user, and SOCK_RDM, which is planned, but not yet implemented, are not described
       here.

       The  protocol specifies a particular protocol to be used with the socket.  Normally only a
       single protocol exists to support a particular socket type within a given protocol family.
       However, it is possible that many protocols may exist, in which case a particular protocol
       must be specified in this manner.  The protocol number to use is particular to the "commu-
       nication domain" in which communication is to take place; see protocols(3N).

       Sockets	of  type  SOCK_STREAM  are  full-duplex byte streams, similar to pipes.  A stream
       socket must be in a connected state before any data may be sent or received on it.  A con-
       nection	to another socket is created with a connect(2) call.  Once connected, data may be
       transferred using read(2) and write(2) calls or some variant of the  send(2)  and  recv(2)
       calls.	When  a session has been completed a close(2) may be performed.  Out-of-band data
       may also be transmitted as described in send(2) and received as described in recv(2).

       The communications protocols used to implement a SOCK_STREAM insure that data is not  lost
       or  duplicated.	If a piece of data for which the peer protocol has buffer space cannot be
       successfully transmitted within a reasonable length of time, then the connection  is  con-
       sidered	broken and calls will indicate an error with -1 returns and with ETIMEDOUT as the
       specific code in the global variable errno.  The protocols optionally keep sockets  "warm"
       by  forcing transmissions roughly every minute in the absence of other activity.  An error
       is then indicated if no response can be elicited on an otherwise  idle  connection  for	a
       extended period (e.g. 5 minutes).  A SIGPIPE signal is raised if a process sends on a bro-
       ken stream; this causes naive processes, which do not handle the signal, to exit.

       SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM sockets.  The only dif-
       ference	is  that  read(2)  calls  will	return only the amount of data requested, and any
       remaining in the arriving packet will be discarded.

       SOCK_DGRAM and SOCK_RAW sockets allow sending of  datagrams  to	correspondents	named  in
       send(2)	calls.	Datagrams are generally received with recvfrom(2), which returns the next
       datagram with its return address.

       An fcntl(2) call can be used to specify a process group to receive a  SIGURG  signal  when
       the  out-of-band data arrives.  It may also enable non-blocking I/O and asynchronous noti-
       fication of I/O events via SIGIO.

       The operation of sockets is controlled by socket level options.	These options are defined
       in  the	file  <sys/socket.h>.	Setsockopt(2)  and  getsockopt(2) are used to set and get
       options, respectively.

RETURN VALUE
       A -1 is returned if an error occurs, otherwise the return value is a descriptor	referenc-
       ing the socket.

ERRORS
       The socket call fails if:

       [EPROTONOSUPPORT]   The	protocol  type	or the specified protocol is not supported within
			   this domain.

       [EMFILE] 	   The per-process descriptor table is full.

       [ENFILE] 	   The system file table is full.

       [EACCESS]	   Permission to create a socket of the specified type and/or protocol is
			   denied.

       [ENOBUFS]	   Insufficient  buffer space is available.  The socket cannot be created
			   until sufficient resources are freed.

SEE ALSO
       accept(2),  bind(2),  connect(2),  getsockname(2),  getsockopt(2),  ioctl(2),   listen(2),
       read(2), recv(2), select(2), send(2), shutdown(2), socketpair(2), write(2)
       ``An  Introductory  4.3BSD Interprocess Communication Tutorial.''  (reprinted in UNIX Pro-
       grammer's Supplementary Documents Volume 1, PS1:7) ``An Advanced 4.3BSD Interprocess  Com-
       munication  Tutorial.''	(reprinted in UNIX Programmer's Supplementary Documents Volume 1,
       PS1:8)

4.2 Berkeley Distribution		   May 23, 1986 				SOCKET(2)
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